Electric power generation and distribution systems, especially those employed for use on aircraft, are required to provide certain redundancies to ensure that the loss of a single source of electric power does not result in a total loss of electric power to the utilization equipment. One method of providing this capability is to utilize at least two sources of electric power driven by separate prime movers, and a distribution system which allows any source of electric power to be coupled to any load, such as is illustrated in FIG. 1. By utilizing such a system, any source fault or change in availability of a source of electric power will result only in the transfer of loads to another source of electric power. Additionally, a power source priority scheme is included which ensures that as higher priority power sources become available, the utilization equipment loads will be transferred thereto and powered therefrom. As an example of the power source priority scheme, if the utilization equipment is first powered by the external power, they will be transferred to the APU generator when the APU becomes available, and then again to the main engine generators when the main engines are started.
Typically, a short break in power delivered to the loads results from the transfer from one source to another as illustrated by FIG. 2. This break is a result of opening the present source's contactor prior to closing the target source's contactor to complete the transfer. This method of break power transfer greatly simplifies the control needed to transfer loads to different sources because the two sources are never electrically coupled one to the other. Each transfer simply entails turning off the present source, then turning on the target source.
A problem is realized with this simplistic approach of power transfer, however, as more and more electronic and computer equipment is utilized. As discussed above, each break power transfer results in a total loss of electric power to the utilization equipment for a short duration. Although the control of the transfer attempts to keep this break to a minimum, the duration of power loss is controlled largely by the mechanical actuation time of the contactors themselves. As a result, the break in power during a power source transfer may be of sufficient duration to cause the electronic and computer equipment to reset. While this may not be a significant problem on ground based distribution systems, on airborne systems a computer memory reset, which may take from two to 10 seconds, while in flight could result in significant problems for the flight crew.
To resolve any potential problems which may have resulted from the use of a simplistic break power transfer system, modern electric power generation and distribution systems utilize no-break power transfer systems. For a no-break power transfer, as illustrated in FIG. 3, the target power source's contactor is closed prior to opening the present power source's contactor so that the utilization equipment never is exposed to a break in electric power. During this power transfer, the two sources of electric power are electrically coupled together for a short duration such that any difference in voltage, frequency, or phase will result in power circulating between them. To minimize this circulating power, the duration that the two sources are electrically coupled together and the difference in voltage, frequency, and phase are all controlled to a minimum prior to initiating the transfer by closing the target source's contactor.
Even small differences in these parameters, however, will result in large currents circulating between the sources during the transfer due to the low impedance of the feeders coupling the sources together. As an example, a phase error of only 3.degree. between the source voltages will result in approximately 260 Amperes of circulating current calculated as follows: ##EQU1##
While this amount of circulating current may be handled by larger conventional rotating generators, many electric power generating systems utilize static sources of electric power which cannot easily dissipate this much power. A common topology of the static power converter 100 utilizes a DC link 102 and a DC link capacitor 104 as illustrated by FIG. 4. A series of power semiconductor switches Q1-Q6 then convert this dc link power into a poly-phase controlled frequency ac output on feeders 106-108 for use by the utilization equipment. These switches, as with all semiconductor components, can be damaged by exposing them to excessive voltage. During the no-break power transfer event with a 3.degree. phase difference between its output voltage and the other source's output voltage, the circulating current will flow from the feeders 106-108, through the flyback diodes D1-D6 and to the dc link capacitor 104. This current flow will result in a net energy transfer from the distribution system to the dc link, resulting in an increase in the dc link voltage. This dc link voltage increase, in turn, may well be enough to result in severe damage to the power semiconductor switches Q1-Q6 of the static source 100.
To preclude the occurrence of this problem, conventional no-break power transfer sensing controls monitor a single phase of the old power source and the same phase of the new power source. When the phase error between these two phases is less than a set threshold, the control allows the transfer to take place. This method of enabling no-break power transfers, while acceptable when transferring between two rotating sources, may still result in damage to a static source such as the converter described above. The problem is that due to unbalances in the loading of the phases of the generator, the three phase output power cannot be assumed to be perfectly 120.degree. displaced one from another. Additionally, machine winding tolerances may also account for a small deviation in the phase displacement of the rotating machine. The output of the electronic converter, however, is controlled to generate three phases which are perfectly displaced by 120.degree. one from the other. If only one phase of the rotating generator were sensed and if the three phases of that machine were not perfectly 120.degree. displaced, a no-break power transfer would be allowed when the associated phase of the converter becomes synchronized with it. If the 120.degree. symmetry varied, as demonstrated above, by even a difference of only 3.degree., large potentially damaging circulating currents will result.
The instant invention is directed at overcoming this problem and allowing only those no-break power transfers which will not result in damage to a static source.